An impressive number of P2 receptors subtypes has been cloned since 1993. A new molecular nomenclature has then been created in which G protein-coupled P2 receptors have been named P2Y while P2 receptors having an intrinsic ion channel activity have been named P2X. The P2Y family encompasses selective purinoceptors (the P2Y1 receptor activated by ATP and ADP), nucleotide receptors responsive to both adenine and uracil nucleotides (P2Y2 receptor: activated equipotentally by ATP and UTP) and pyrimidinoceptors (the P2Y3 and P2Y6 receptors activated by UDP; the P2Y4 receptor: activated by UTP). The P2Y5 and P2Y7 receptors display limited homologies with the other members of the P2Y family. They have been included in this family especially on the basis of radioligand binding studies showing affinities for adenine nucleotides (1-18).